Studies have been initiated to investigate the relationships between vascular hyperreactivity, abnormal catecholamine metabolism, and the renin-angiotensin system in patients with essential and secondary hypertension. Initially, the assessment of renin activity in plasma was carried out by a bioassay procedure. In the past year, a precise, sensitive, and specific radioimmunoassay for angiotensin I has been developed and adapted to the measurement of both plasma renin activity (PRA) and plasma renin substrate (PRS). The radioimmunoassay requires determination of I125, a gamma emitting isotope, by use of a gamma scintillation system. In order to continue and extend the use of the radioimmunoassay of angiotensin I for measurement of PRA and PRS, support for a full-time gamma scintillation system is requested.